This invention relates to a technique utilized in the fabrication of microminiature devices and, more particularly, to a method and apparatus for measuring the widths of features on masks and wafers employed in making large-scale-integrated (LSI) devices.
It is well recognized that highly accurate control of the size of features formed on masks and wafers is critical in the overall process of manufacturing reliable LSI devices. Thus, for example, before using a set of masks to form features on an associated wafer, it is important that each of the masks be checked to determine whether or not specified linewidths thereon have been made within prescribed tolerances. And, subsequently in the process, it may be important to check the widths of features actually formed on the wafer.
A number of techniques are known in the art for measuring linewidths. Each such technique has its own unique set of characteristics which make it suitable or not for a particular application. Thus, for example, the conventional technique known as image shearing is characterized by medium cost and general utility. But image shearing is relatively slow and exhibits only fair reproducibility because it requires that an operator make judgments as to when cross hairs or split images are exactly aligned.
In making LSI devices by contact or near-contact lithographic processes, mask degradation is a common problem. Accordingly, plural replicas of each mask of the set of masks required for a particular device must typically be available. As a result, the number of masks utilized to fabricate some devices of practical interest becomes relatively large. With known techniques, the monitoring of such a large number of masks to assess whether of not their linewidths are satisfactory is a time consuming and expensive operation.
Accordingly, the need arose for a linewidth measurement technique that would be fast, accurate and relatively low cost. It was recognized that such a technique, if available, would facilitate the fabrication of reliable LSI devices.